"Donald Trump is a phony, a fraud," said Romney, the GOP's unsuccessful 2012 nominee for president. "His promises are as worthless as a degree from Trump University."

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In taking aim at Trump, Romney said the current front-runner lacks the temperament and integrity to be president

Romney said the only serious policy proposals that deal with the broad range of today's national challenges come from Texas Sen. Ted Cruz, Florida Sen. Marco Rubio and Ohio Gov. John Kasich.

Trump responded, and called Romney a failed candidate and called his comments irrelevant.

"That was a race, folks, I have to say should have been won," Trump said.

People began lining up early Thursday morning for a chance to attend the Trump rally despite the low temperatures.

"I really like that the lobbyists can't buy him, and I like that he's a businessman, and I feel like we need a businessman to run the country," said Prentiss Kurtz of West Paris.

Along with supporters, a large group of protesters gathered outside the hotel Thursday morning. People held signs that said "Dump Trump" and "Love always trumps hate."

Julia Legler of Portland, added: "We have a lot of new Mainers here, and we want to show them that they're welcome, and people like Trump are really not saying that; in fact, they're saying the complete opposite of that."

But inside, Trump continued to attack Romney.

"I backed him," Trump said. "You can see how loyal he is. He was begging for my endorsement. I could have said, 'Mitt, drop to your knees.' He would have dropped to his knees. He was begging."

Trump also said that he warned Romney to stay out of the race this year.

"Mitt was all set to run," Trump said. "I know this from people that are close to him, and I think he probably still has a desire maybe at the convention to try and get some kind of thing. Hillary Clinton will destroy him in the election assuming she's allowed to run ... assuming she's not arrested for the email situation."